1. Field of the Invention
This invention relates to a process for forming a functional deposited film useful as functional films, such as thin film transistors (TFT), thin film diodes, phototransistors, photodiodes, or IC and LSI in which these are integrated, or semiconductor devices such as solid state image pick-up devices, light inputting devices, etc., and also devices for such process therefor.
2. Description of related art
For example, a silicon thin film which is one of functional deposited films to be utilized for semiconductor devices, etc., has excellent electric or photoelectric characteristics and a wide scope of applications of promising future use.
Such silicon thin films may be morphological classified broadly into those of polycrystalline phase and those of amorphous phase and are different in various characteristics.
For such silicon thin film, it has been desired and investigated in recent years to establish a technique for separately preparing a silicon thin film of polycrystalline phase and a silicon thin film of amorphous phase on the same substrate. For example, for a photosensor, it is desirable to form a light-receiving section constituting the sensor section at a predetermined position on a substrate with the use of an amorphous silicon thin film having great photosensitivity and a transfer section constituting the signal processing section at a different position above on the substrate with the use of a polycrystalline silicon thin film having great charge mobility.
However, it has been difficult to prepare a silicon thin film of both polycrystalline and amorphous phase at the separate desired positions on the same substrate according to the CVD (Chemical Vapor Deposition) method such as the plasma decomposition CVD method utilizing glow discharging or the thermal CVD method, etc., or the sputtering method, etc.
For example, in the plasma decomposition CVD method, starting gasses of monosilane (SiH.sub.4), disilane (Si.sub.2 H.sub.6) or these gases diluted with H.sub.2, Ar, He gas, etc., are used and silicon thin films are formed on a desired substrate by decomposing these gases utilizing glow discharging. For example, in the case of a substrate temperature of 300.degree. C. and discharging power of 0.1 W/cm.sup.2, under the conditions of deposition with a volume ratio of 100 to 10% of SiH.sub.4 relative to H.sub.2, an amorphous silicon thin film is formed. In contrast, under greater glow discharging power and smaller volume ratio of SiH.sub.4 to H.sub.2, an amorphous silicon thin film containing fine crystalline phase (polycrystalline phase with small grain size) is formed in the film. Such a silicon thin film has excellent doping properties and great charge mobility, but the distribution of the amorphous phase and the microcrystalline phase in the silicon thin film is microscopic in nature, whereby it is impossible to prepare at the same time a silicon thin film of amorphous phase and a silicon thin film of polycrystalline phase by designating regions with desired sizes on a substrate.
As the method for preparing a silicon thin film of amorphous phase and a silicon thin film of polycrystalline phase by sequential plasma decomposition CVD method on a single substrate, it is conceivable to first deposit a polycrystalline silicon thin film on the substrate under appropriate conditions, removing unnecessary portion by means of etching, etc., then depositing an amorphous silicon thin film under appropriate conditions and removing unnecessary portion by means of etching, etc. However, according to this method, the steps are cumbersome and also, it is difficult to effect etching selectively the amorphous silicon thin film and the polycrystalline silicon thin film, whereby there is involved the problem with respect to reliability of the steps.
According to the thermal CVD method, radicals are formed by pyrolysis of a starting material such as monosilane or sisilane, etc., and a silicon thin film is formed by attaching these radicals onto a substrate. Also, in this method, although an amorphous silicon thin film and a polycrystalline silicon thin film can be prepared by selecting appropriate deposition conditions, it is impossible to separately prepare these films at the same time on a single substrate. On the other hand, it has been proposed to irradiate with laser beam or other light beam the whole or a part of an amorphous silicon thin film deposited by the above plasma decomposition CVD method or the thermal CVD method to make such irradiated portion polycrystalline. Although this method has the advantage of making only the minute region selectively polycrystalline by effecting small spot irradiation with a throttled light beam by use of an appropriate optical system, the following problems are involved under the present situation. That is, thermal damages may be caused on the substrate or around the irradiated portion depending on the irradiation conditions, and polycrystalline thin film of good quality cannot be obtained, because it is difficult to make an amorphous silicon thin film containing hydrogen polycrystalline by light irradiation. With respect to the latter problem, the quality of the polycrystalline thin film obtained by light irradiation can improved by suppressing the hydrogen content in the amorphous silicon thin film to a low level, but the film quality of the amorphous silicon thin film will be lowered corresponding to the decreased content of hydrogen. Also in this method, it is difficult to separately prepare an amorphous silicon thin film with good film quality and polycrystalline silicon thin film with good film quality separately on a single substrate. These same problems are also involved with other materials such as germanium or carbon.